1. Field of the Invention
The present invention is directed to communication systems and networks and is particularly directed toward methods for controlling shared access to wireless transmission systems.
2. Background of Related Art
Network systems and network traffic loads have evolved and stratified in several important dimensions. The systems have adapted to a great range of distancesxe2x80x94e.g., from personal area networks (PANs) to local area (LANs), metro area (MANs) and wide area (WANs). The bandwidth properties of the physical media have also grown to span a wide range of possibilitiesxe2x80x94there are significant transmission systems for kilobits, megabits, gigabits, and terabits. Traffic loads have grown from simple telecommunication and file transfer applications to include a wide variety of traffic types such as:
bursty asynchronous small data (e.g., 1-128 byte messages);
bursty asynchronous file/page data (e.g., 1-16 KB messages);
bursty asynchronous bulk data (e.g., 1 MB and beyond);
streaming (isochronous) small data, file data and bulk data;
streaming (variable bit rate);
multicast or broadcast data;
guaranteed rate and/or guaranteed jitter; and
prioritized traffic.
It is important to consider how diverse traffic types can coexist on a given network or mix of networks. The effectiveness of different methods for carrying different kinds of traffic will vary with the geographic range of the networkxe2x80x94i.e., PAN, LAN, MAN, WANxe2x80x94with the available bandwidth on the medium, with the control scheme of the network, and with the mix of the traffic types loading the network.
Many networking systems, e.g., Ethernet, were invented prior to the existence of a wide range of traffic types. Ethernet pioneered packet transmission over a contention medium and is not optimized for isochronous traffic. Other networks have been designed for different missions. Examples include Fibre Channel which was originally developed as an interconnect method for disk storage, and ATM which was originally designed for telephony and media traffic. Wireless networks, while sharing the wireless medium, have been developed with great diversity including both cell-based TDMA control (HiperLan) and Ethernet-like contention-based methods (IEEE 802.11).
Contemporary wireless communication systems specify one or more communication xe2x80x9cchannelsxe2x80x9d, or frequency bands, for stations to transmit and receive encoded data. The channels are used in one of two ways: a station transmits data to a control unit, usually called an access point (AP), which forwards the data to another station (forwarding mode), or a station may transmit data directly to a destination station without passing through an access point (direct mode).
Forwarding mode provides an advantage in that stations that may not be able to transmit directly to each other because of range limitations or other problems can still communicate by forwarding through the access point. One disadvantage of this method is that data must traverse the channel twice, thus reducing the total available bandwidth by half.
The access point in wireless networking supplies control methods and protocols that coordinate the various transfers between wireless stations. It is common practice to define an ad hoc network as a set of wireless stations without a dedicated access point. In this configuration it is assumed that some, if not all, of the stations are capable of serving as access point and that a selection procedure exists whereby one of the stations will provide the necessary control functions.
The IEEE 802.11a standard specifies multiple channels each consisting of multiple carrier frequencies with several possible modulation schemes, e.g., OFDM, defined for the channels. In typical practice a channel is operated as a monolithic unit where a transmitter always sends on the complete set of carriers defined for a channel and a receiver always receives from the complete set of carriers. New technologies have been invented whereby a transmitter can use a subset of the carriers while other transmitters simultaneously utilize a different non-conflicting set of carriers from the same channel. This technology introduces a new operational mode for the channel which can be called xe2x80x9coverlaid modexe2x80x9d. Thus, multiple transmitters can be using a channel in overlaid mode whereas in the normal non-overlaid mode only a single transmitter can be active.
With the addition of an overlaid mode, conventional channels that would otherwise have only two modes (forwarding or direct) could have at least four modes: forwarding, forwarding+overlaid (where the transmitter communicates with the receiver via the access point over only a subset of the full carrier set), direct, and direct+overlaid (where the transmitter communicates with the receive directly over only a subset of the full carrier set).
Wireless networks and particularly networks with multi-carrier channels have certain constraints:
radios used in wireless networking are not able to simultaneously send and receive (because the transmitter would overwhelm a local receiver); and
power received on different carriers in a multi-carrier system must be approximately equal across all the carriers; otherwise signals on stronger carriers will overwhelm weaker carriers in systems where a common amplifier with automatic gain control processes all of the carriers.
There are a number of factors such as uniform timing, frequency stability, multi-path and noise phenomena that are preferably considered as well in creating a viable overlaid transmission system. The issue of power control in the overlaid mode is another of these. There exist both open-loop and closed-loop power control methods that help a single transmitter adjust its power output, and help a single receiver adjust its receiver gain, when a particular transmitter/receiver pair is active. These procedures are not applicable when either
multiple transmitters are sending to the same receiver at the same time; or
multiple transmitters are engaged in direct transfers in an overlaid mode at the same time.
A wireless transmission system experiences much higher bit error rates, or packet error rates, than a comparable wired transmission system. In order to make wireless systems robust the physical layer design of wireless systems typically incorporates some or all of the following adaptive techniques:
means to select a more robust, and hence lower data rate, encoding scheme when the channel is noisy or a less redundant and a higher data rate encoding scheme when the channel is clear;
means to quickly acknowledge the correct reception of a packetxe2x80x94and thus to quickly retransmit the packet if necessaryxe2x80x94in order to reduce the packet error rate; and
means to change frequencies or channels in a dynamic manner in order to move from a frequency band that is noisy or affected by multi-path effects to a different frequency band that is better.
These techniques for improving channel robustness must be reconsidered, modified, or replaced in the context of overlaid communication. The frequency assignment problem is more complicated because multiple carriers must be assigned to multiple transmitters instead of all carriers being assigned to a single transmitter. The traditional method of acknowledging a transmitted packet must be reconsidered in overlaid mode because there are multiple simultaneous packets to acknowledge rather than a single packet. Mobile stations may need to change carrier frequencies, but it will be difficult to determine which frequencies are available and which frequencies might offer some improvement.
Access methods for wireless channels fall into three general categories: contention methods, polling methods, and TDMA methods. Contention systems such as IEEE 802.11 use heuristicsxe2x80x94e.g., random backoff, listen-before-talk, and mandated interframe delay periodsxe2x80x94to avoid (but not completely eliminate) collisions on the wireless medium. IEEE 802.11 also employs a beacon message which can be asserted by the access point and which allows the access point to individually poll selected stations for sending or receiving data. The duration of the polling period is controlled by a parameter set by the access point and contained within the beacon message. Slotted systems, e.g., TDMA, assign timeslots to individual transmitters to eliminate collision and assign predictable amounts of bandwidth. The contention systems are well-suited to asynchronous bursty traffic. These systems work particularly well when the burst sizes are comparable to the natural packet size of the medium, or small multiples of the natural packet size. Slotted systems are well-suited to isochronous applications that have a need for continuous channel bandwidth, although they may have extra overhead in comparison to contention systems when carrying asynchronous bursty traffic.
Methods have been devised to map different kinds of traffic to a particular medium; i.e., to give slotted media some of the attributes of contention media and vice versa. For example, Fibre Channel classes and ATM adaptation layers or convergence layers specify procedures for mapping different kinds of traffic onto underlying media. In all cases these are mappings onto an underlying mediumxe2x80x94packet-based in the case of Fibre Channel or cell-based in the case of ATM. Mappings and convergence layers are separate and distinct from the underlying physical medium, and the distinction is equally appropriate for wireless network which have been designed to operate either as contention networks or slotted or TDMA networks.
In view of the above problems of the art, it is an object of the present invention to provide a wireless transmission system capable of many operational modes, e.g., forwarding or direct; overlaid or non-overlaid; contention, polled or slotted; acknowledged or non-acknowledged.
It is another object of the present invention to provide a method for operating a physical channel in several modes.
It is a further object of the present invention to provide a method for calibrating transmit power levels in an overlaid multi-carrier system with changing conditions or mobile stations.
It is a further object of the present invention to provide a method for comparing signal quality of different frequencies in order to select carriers for overlay operation in a multi-carrier system.
It is yet another object of the present invention to provide a method for supporting direct station-to-station communication in a multi-carrier system with overlaid capability.
It is a still further object to provide a method for improving the efficiency of the medium regarding control messages such as acknowledgements
It is another object of the present invention to provide a method for adjusting power levels and frequency assignments in an overlaid or non-overlaid system for mobile stations or for stations that experience changes in their local environment.
The above objects are met according to a first aspect of the present invention by providing techniques for controlling shared access to wireless communication systems. A wireless communication system implements a calibration mode by which individual nodes in the system can determine which other nodes in the system are physically close to them and therefore can be reached with less than full transmitting power. It may also implement a calibration mode by which the signal quality at different carrier frequencies can be determined for pairs of stations. Nodes which can communicate with one another via low power can form a low power constellation (a subset of the complete network) whose nodes can communicate directly with one another using this low power arrangement. The direct communication mode may additionally be used by itself. In another mode, the nodes in the system can communicate amongst themselves via bridgesxe2x80x94other, non-access point nodesxe2x80x94to lessen the load on the access point or to accommodate environmental or other conditions. Various ones of these modes may be combined into a predetermined cycle of communication modes to help the physical layer accommodate various types of data handled by the network using beacons or another coordinating technique. Overhead in the form of packet retransmission may be reduced by interpolating to recover lost packets rather than retransmitting them.